The fasting mimicking diet (FMD): Longo, ProLon, and what the trials report

The fasting mimicking diet (FMD) is a 5-day eating pattern designed to produce metabolic effects similar to water-only fasting while providing some nutrition. Developed by Valter Longo at USC, it's the most-studied multi-day caloric restriction protocol in the longevity space.

The macronutrient design#

The 5-day FMD is a specific composition designed to keep insulin, IGF-1, and amino-acid-sensing pathways suppressed:

• Day 1: ~1,100 kcal, predominantly plant-based, low protein (~10% kcal), moderate fat
• Days 2–5: ~750 kcal/day, same composition
• All days: very low animal protein, no added sugar, no sweetened beverages
• High polyunsaturated fat and complex carbohydrate from specific plant sources

The published FMD (and the commercial ProLon product) provides specific foods (soup bases, nut bars, olives, herbal teas). ProLon is marketed as a medical food; medical foods are a defined food category, not drugs, and the FDA does not review or approve them before sale.

Why it's not just a calorie-restriction diet#

The argument for the specific composition is that very-low-protein + low-glycemic carbohydrate + moderate fat produces the IGF-1 suppression, ketosis, and growth-factor signalling changes characteristic of true fasting, without complete food deprivation.

The blood biomarker shifts in trials support this. After 5 days of FMD, healthy adults show:

• IGF-1 decreased ~25%
• Ketone bodies elevated
• Glucose decreased
• Hs-CRP decreased
• Various stem cell and immune markers altered (more in animal data than humans)

What the trials show in humans#

Brandhorst et al. 2015: mostly a mouse and yeast study, with a small human pilot trial of 19 adults on the FMD versus 19 controls for 3 cycles. Results in the pilot: about a 3% reduction in body weight and lower IGF-1, among other risk-factor changes, without major adverse effects.

Wei et al. 2017: a larger randomised trial of 100 adults, 71 of whom completed three FMD cycles. Results: body weight fell by about 2.6 kg, along with reductions in trunk and total body fat, blood pressure, and IGF-1. C-reactive protein improved only in participants who were already at risk for disease.

Caffa et al. 2020 — adjunctive use in breast cancer patients receiving hormone therapy; small human sub-study; reduced insulin, leptin, and IGF1 levels, mirroring the mouse findings.

FMD vs. continuous calorie restriction. Where head-to-head comparisons exist, FMD shows broadly similar weight outcomes to matched-calorie continuous restriction. The proposed mechanistic advantages (deeper fasting signalling) are real biochemically; whether they translate to long-term outcome differences in humans is not yet established.

Where claims outrun evidence#

• Cancer treatment. Longo's lab has substantial mouse evidence on FMD + chemotherapy. Human evidence is small adjunct trials, not curative claims.
• Autoimmune disease. Animal data are intriguing; human evidence is small.
• Longevity in humans. No human lifespan or hard-endpoint data. The biomarker data suggest plausible benefit; translation to outcomes isn't established.
• Regeneration claims. Mouse studies showing stem-cell-driven regeneration with FMD cycles have not been replicated cleanly in human tissue.

The published human evidence supports modest cardiometabolic improvements with periodic 5-day FMD cycles. The lifespan and disease-prevention claims are extrapolation.

ProLon: the commercial reality#

ProLon is the commercial 5-day kit from Longo's company L-Nutra. It's expensive (typically £150–£250 per cycle). The protocol is the FMD; the price is the convenience of pre-packaged compliance.

You can replicate the FMD composition from regular food at much lower cost. Several papers and Longo's own materials have published the macronutrient targets; building a 5-day plan from soups, nuts, vegetables, and olive oil is achievable for someone willing to do the work.

Safety considerations#

FMD is safer than water-only fasts of equivalent duration because of the partial caloric and electrolyte intake. The same contraindications still apply:

• Pregnancy or breastfeeding
• Diabetes on insulin or sulfonylureas without clinical supervision
• BMI ≤18.5
• Eating disorder history
• Children and adolescents
• Significant cardiac or renal disease
• Frailty in older adults

Most healthy adults tolerate 5-day FMD cycles with mild symptoms (hunger, fatigue, headaches) that resolve quickly on refeeding.

What we'd actually say#

For someone interested in periodic deeper fasting without the safety considerations of water-only multi-day fasting, the FMD is a reasonable evidence-based protocol with modest biomarker effects in trials.

For most readers, 3–4 cycles per year of FMD (Longo's recommended cadence in low-risk adults) is the published rhythm. The commercial product is convenient; the protocol is replicable from regular food.

The longevity hype around FMD outpaces the human evidence. Don't expect lifespan extension; do expect modest cardiometabolic improvements that partially reverse without continued cycles.

When to talk to a clinician#

• Diabetes, especially on insulin or sulfonylureas
• BMI ≤18.5 or recent unintentional weight loss
• Pregnancy or breastfeeding
• Active cancer treatment: adjunct use should be specialist-supervised, not self-administered
• Significant cardiac or renal disease
• Eating disorder history